test/librbd: add LengthModifiedDiscardJournalAppendEnabled test

test/librbd: add LengthModifiedDiscardJournalAppendEnabled test

Currently nothing triggers the length_modified case in
ImageDiscardRequest::prune_object_extents() in isolation. It's only
triggered in DiscardGranularityJournalAppendEnabled test together with
the prune_required case and a bad refactoring could easily break the
length_modified logic again.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 34e59c450298e1c05f07570f74a9489bd8479a44)

Conflicts:
	src/test/librbd/io/test_mock_ImageRequest.cc [ commit
	  b9a2384cdc43 ("librbd: propagate area down to
	  file_to_extents()") not in quincy ]